linux-kernel - Re: [RFC PATCH v7 1/7] Restartable sequences system call

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Date:	Wed, 3 Aug 2016 13:29:35 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Ben Maurer <bmaurer@...com>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call

On Tue, 26 Jul 2016, Mathieu Desnoyers wrote:

> > What problem does this solve?
>
> It allows user-space to perform update operations on per-cpu data without
> requiring heavy-weight atomic operations.


This is great but seems to indicate that such a facility would be better
for kernel code instread of user space code.

> First, prohibiting migration from user-space has been frowned upon
> by scheduler developers for a long time, and I doubt this mindset will
> change.

Note that the task isolation patchset from Chris Metcalf does something
that goes a long way towards this. If you set strict isolation mode then
the kernel will terminate the process or notify you if the scheduler
becomes involved. In some way we are getting that as a side effect.

Also prohibiting migration is trivial form user space. Just do a taskset
to a single cpu.